The present invention relates, in general, to semiconductor devices, and more particularly, to semiconductor devices formed on an opposed leadframe.
Optoisolators are electronic components which transmit electrical information between a light source (emitter) and a light detector (receiver), without electrical connection. The light emitted can be either invisible such as infrared or can be emitted in the visible spectrum. An enabling input signal to the optoisolator causes the light source to turn on. The light detector then senses the light from the emitter source and produces a corresponding output signal. As a result, the output signal is coupled to the input signal optically rather than with an electrical connection. Consequently such devices are called optoisolators. In some electrical circuit applications, a high voltage circuit and a low voltage circuit must interact in order to process data. To electrically isolate the two circuits from noise or cross talk, optoisolators are used.
In order for an optoisolator to operate properly, the light source and light detector must be oriented properly with respect to each other. If too little light from the emitter reaches the detector, then the sensitivity of the detector suffers. If too much light reaches the detector, then the speed performance suffers because saturation of the phototransistor on the detector will reduce the switching speed of the detector. If the spacing between the emitter and the detector is too great, light transfer efficiency will be decreased and if the distance is too short, the isolation voltage of the optoisolator is degraded.
To accurately control the placement of the emitter component relative to the detector component, previously known methods for assembling optoisolators have placed the two electronic components on separately formed lead frames and then molding them together while maintaining the proper alignment and spacing. In an effort to reduce the cost of a two leadframe system, other techniques place both components on the same leadframe and then encapsulate both components in a transparent mold which is covered with a reflective shield. Even more complicated methods attach the two components on the same leadframe, but prior to encapsulating the devices, a portion of the leadframe containing one device is bent 180.degree. such that the two devices are folded on top of each other. All of these techniques either require additional material cost or introduce complexity into the assembly process which in turn will increase the final cost of the optoisolator. Due to the malleability of the rotated leadframe, this method can have variation in the final placement of the two devices. This will degrade the performance of the optoisolator.
Accordingly, it would be advantageous to have a method for forming a semiconductor device such as an optoisolator from a single leadframe. It would be of further advantage if the method did not require the formation of a reflective coating layer and could provide a method that could accurately control and repeat the spacing between the electronic elements of the semiconductor device.